By docking or toilets (Futa/Inflation/Body Expansion)

"... Inside a futa's dick or use a bathroom.

In this reality men are rare, so most women grown both genitals, so Futa are many times more fertile and need a lot of biomass to help produce sperm. A Futa inserts their dick into the urethra of another and unloads their pee and poop into the other Futa's testicles. Once full, the excrement is dissolved into semen. But there very few are occasions where they take in too much and is distributed to either the tits to be turned into breast milk or emptied into a toilet. It's so rare that they eat too full due to having rubber-like skin that can stretch to a ridiculous degree...

Although, there have been a few occasions where they take in so much that they become balloons. There has been a rumor their neighbor actually burst like a water balloon, but since she was seen earlier that day, there's no way that's true. Unless she regenerates...